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Message-ID: <CANn89iJD6ZYCBBT_qsgm_HJ5Xrups1evzp9ej=UYGP5sv6oG_A@mail.gmail.com>
Date: Wed, 2 Jul 2025 07:02:27 -0700
From: Eric Dumazet <edumazet@...gle.com>
To: Jiayuan Chen <jiayuan.chen@...ux.dev>
Cc: netdev@...r.kernel.org, mrpre@....com, 
	Neal Cardwell <ncardwell@...gle.com>, Kuniyuki Iwashima <kuniyu@...gle.com>, 
	"David S. Miller" <davem@...emloft.net>, David Ahern <dsahern@...nel.org>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, 
	David Howells <dhowells@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next v1] tcp: Correct signedness in skb remaining
 space calculation

On Wed, Jul 2, 2025 at 6:59 AM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Wed, Jul 2, 2025 at 6:42 AM Jiayuan Chen <jiayuan.chen@...ux.dev> wrote:
> >
> > July 2, 2025 at 19:00, "Jiayuan Chen" <jiayuan.chen@...ux.dev> wrote:
> >
> >
> > >
> > > The calculation for the remaining space, 'copy = size_goal - skb->len',
> > >
> > > was prone to an integer promotion bug that prevented copy from ever being
> > >
> > > negative.
> > >
> > > The variable types involved are:
> > >
> > > copy: ssize_t (long)
> > >
> > > size_goal: int
> > >
> > > skb->len: unsigned int
> > >
> > > Due to C's type promotion rules, the signed size_goal is converted to an
> > >
> > > unsigned int to match skb->len before the subtraction. The result is an
> > >
> > > unsigned int.
> > >
> > > When this unsigned int result is then assigned to the s64 copy variable,
> > >
> > > it is zero-extended, preserving its non-negative value. Consequently,
> > >
> > > copy is always >= 0.
> > >
> >
> > To better explain this problem, consider the following example:
> > '''
> > #include <sys/types.h>
> > #include <stdio.h>
> > int size_goal = 536;
> > unsigned int skblen = 1131;
> >
> > void main() {
> >         ssize_t copy = 0;
> >         copy = size_goal - skblen;
> >         printf("wrong: %zd\n", copy);
> >
> >         copy = size_goal - (ssize_t)skblen;
> >         printf("correct: %zd\n", copy);
> >         return;
> > }
> > '''
> > Output:
> > '''
> > wrong: 4294966701
> > correct: -595
> > '''
>
> Can you explain how one skb could have more bytes (skb->len) than size_goal ?
>
> If we are under this condition, we already have a prior bug ?
>
> Please describe how you caught this issue.

Also, not sure why copy variable had to be changed from "int" to "ssize_t"

A nicer patch (without a cast) would be to make it an "int" again/

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